Character Controller run not working

I followed a Brackeys tutorial to make fps player movement. It works fine but when I try to add an option to run then it won't let me run. Anyone know how I can fix this?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerMovement : MonoBehaviour
 {
     // Movement variables
     public CharacterController controller;
     public float speed;
     public float walkSpeed = 12f;
     public float runSpeed = 20f;
 
     // Gravity variables
     Vector3 velocity;
     public float gravity = -9.81f;
     public Transform groundCheck;
     public float groundDistance = 0.4f;
     public LayerMask groundMask;
     bool onGround;
     public float jumpHeight = 3f;
 
     // Every frame
     void Update()
     {
         // Check if we are on the ground
         onGround = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);
         if (onGround == true && velocity.y < 0)
         {
             // Reset velocity
             velocity.y = -2f;
         }
 
         // Get player input
         float x = Input.GetAxis("Horizontal");
         float z = Input.GetAxis("Vertical");
 
         // Check if we want to run
         speed = walkSpeed;
         if (Input.GetKeyDown(KeyCode.LeftControl) || Input.GetKeyDown(KeyCode.LeftShift))
         {
             speed = runSpeed;
         }
 
         // Turn it into the movement then move the player
         Vector3 move = transform.right * x + transform.forward * z;
         controller.Move(move * speed * Time.deltaTime);
 
         // Check if we want to jump
         if (Input.GetButtonDown("Jump") && onGround == true)
         {
             velocity.y = Mathf.Sqrt(jumpHeight * -2 * gravity);
         }
 
         // Increase the players velocity and then add it to the player
         velocity.y += gravity * Time.deltaTime;
         controller.Move(velocity * Time.deltaTime);
     }
 }